A person, usually a child. That always has the most fashionable infection. They always get the flu, and/or always have a cold. They themselves don't really get sick themselves though. Just low grade symptoms.
On the other hand, if you spend any time close to them, you will most likely then get it.
Often becoming QUITE ill.

That little Mary. She came for a visit, now all our kids AND mom have a wicked fever. She's such a little 'bug truck'.